Jiuxiang Dai

Nature Communications, Published online: 08 July 2022; doi:10.1038/s41467-022-31523-w

Two-dimensional (2D) membranes are emerging candidates for osmotic energy conversion but the trade-off between ion selectivity and conductivity remains the key bottleneck. Here, the authors demonstrate a fully crystalline imine-based 2D polymer membrane capable of combining excellent ionic conductivity and high selectivity for osmotic energy conversion.

Green Energy

In article number 2201667, Douglas S. Galvao, Partha Kumbhakar, Chandra Sekhar Tiwary, and co-workers convert dust crystals of biotite ores from mountains into atomically thin two-dimensional materials using liquid exfoliation. Two-dimensional-biotite can be utilized to harvest small force/pressure or strain for generating green electricity.

Abstract

Understanding the substrate and temperature effect on thermal transport properties of transition metal dichalcogenides (TMDs) monolayers are crucial for their future applications. Herein, a dual-wavelength flash Raman (DF-Raman) method is used to measure the thermal conductivity of monolayer WS₂ at a temperature range of 200–400 K. High measurement accuracy can be guaranteed in this method since the influence of both the laser absorption coefficient and temperature-Raman coefficient can be eliminated through normalization. The room-temperature thermal conductivity of suspended and supported WS₂ are 28.5 ± 2.1 (30.3 ± 2.0) and 15.4 ± 1.9 (16.9 ± 2.1) W/(m·K), respectively, with a ∼ 50% reduction due to substrate effect. Molecular dynamics (MD) simulations reveal that the suppression of acoustic phonons is mainly responsible for the striking reduction. The behaviors of optical phonons are also unambiguously investigated using Raman spectroscopy, and the in-plane optical mode, E _2g ¹ (Γ), is surprisingly found to be slightly enhanced while out-of-plane mode, A_1g(Γ), is suppressed due to substrate interaction, mutually verified with MD results. Our study provides a solid understanding of the phonon transport behavior of WS₂ with substrate interaction, which provides guidance for TMDs-based nanodevices.

Nature Communications, Published online: 07 July 2022; doi:10.1038/s41467-022-31605-9

Here, the authors report the realization of light-emitting field-effect transistors based on van der Waals heterostructures with conduction and valence band edges at the Γ-point of the Brillouin zone, showing electrically tunable and material-dependent electroluminescence spectra at room temperature.

Nature Communications, Published online: 07 July 2022; doi:10.1038/s41467-022-31372-7

Flexible thermoelectrics are of great interest with increasing demand of flexible and wearable electronics. Here, the authors demonstrate that the Weyl semimetal, WTe2, has a high Nernst power factor and great mechanical flexibility.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01314-1

By precisely stacking two sheets of graphene at a specific angle, the resulting moiré superlattice superconducts. Extending this notion, researchers have now found superconductivity in four- and five-layer graphene moiré stacks.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01286-2

The twist angle dependence of correlations in alternating twist quadrilayer graphene is reported.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01287-1

Superconductivity is reported in magic-angle twisted four-layer and five-layer graphene systems. While they find that all magic-angle graphene systems fit into a unified hierarchy of systems that share a set of flat bands in their electronic band structures, they also report that there is a key distinction between magic-angle twisted bilayer graphene and the other family members, related to the difference in the way the electrons move between the layers in a magnetic field.

This article comprises an informative review of advances in the chemical doping of graphene using experimental methods. Different types of chemical doping of graphene, details of doping mechanisms, and recent experimental results for applications in scientific and industrial fields are systematically discussed and summarized. Finally, various challenges, issues, and prospects of research work in this direction are presented.

Graphene has been widely investigated and applied in almost all areas of science and technology. Modulation of graphene properties is needed for its potential utilization in various applications. Chemical doping is one of the most attractive and efficient strategies to alter graphene properties. To date, substantial progress in this area has been reported, which needs to be thoroughly reviewed for its effective implementation in the development of commercial products in the days to come. This article presents a systematic, critical, and more informative review of the recent advancement in the chemical doping of graphene and its applications. Starting with a brief history along with the properties and synthesis of graphene, different experimental approaches to chemical doping and their outcomes reported so far are systematically documented. Further, extensive studies based on potential applications of doped graphene in various fields including light-emitting diodes, photodetectors, solar cells, energy storage devices, sensors, and medical diagnoses are discussed and presented. Finally, the study is concluded with discussions on future prospective research work in this area.

Nature Communications, Published online: 06 July 2022; doi:10.1038/s41467-022-31664-y

The simultaneous achievement of high breakdown voltage and low resistance is a contradictory point because it would require high and low doping simultaneously. Here, Zhou et al. achieve a power figure-of-merit of 13.2 GW/cm2 through hole injection and conductivity modulation effect.

Nature Communications, Published online: 06 July 2022; doi:10.1038/s41467-022-31685-7

The study of the mechanical properties of twisted van der Waals structures can provide important information about their interlayer coupling and electronic behaviour. Here, the authors report a nanoindentation-based technique to determine the interlayer shear stress in bilayer MoS2, showing its independence as a function of the twist angle.

Robust piezoelectricity with spontaneous polarization is detected in tellurene at room temperature due to the strong anisotropic property and dipoles induced by unique charity helical chain structure and strong intramolecular interactions. A charming memory window and high on switching current are realized in tellurene-based memory, providing a new platform to construct robust memory cells for neuromorphic computing.

Abstract

Robust neuromorphic computing in the Big Data era calls for long-term stable crossbar-array memory cells; however, the elemental segregation in the switch unit and memory unit that inevitably occurs upon cycling breaks the compositional and structural stability, making the whole memory cell a failure. Searching for a novel material without segregation that can be used for both switch and memory units is the major concern to fabricate robust and reliable nonvolatile cross-array memory cells. Tellurium (Te) is found recently to be the only peculiar material without segregation for switching, but the memory function has not been demonstrated yet. Herein, apparent piezoelectricity is experimentally confirmed with spontaneous polarization behaviors in elementary 2D Te, even in monolayer tellurene (0.4 nm), due to the highly oriented polarization of the molecular structure and the non-centrosymmetric lattice structure. A large memory window of 7000, a low working voltage of 2 V, and high on switching current up to 36.6 µA µm⁻¹ are achieved in the as-fabricated Te-based memory device, revealing the great promise of Te for both switching and memory units in one cell without segregation. The piezoelectric Te with spontaneous polarization provides a platform to build robust, reliable, and high-density logic-in-memory chips in neuromorphic computing.

Chemically synthesized transition metal dichalcogenide (TMDC) nanowires are exploited as a new type of subwavelength optical nanoresonators to simultaneously support pronounced excitonic and Mie resonances. Strong coupling between excitons and Mie resonances is demonstrated to support Mie-exciton polaritons. Moreover, the optical coupling can be further tuned by optothermal effects in a reversible and reproducible way.

Abstract

Subwavelength optical resonators with spatiotemporal control of light are essential to the miniaturization of optical devices. In this work, chemically synthesized transition metal dichalcogenide (TMDC) nanowires are exploited as a new type of dielectric nanoresonators to simultaneously support pronounced excitonic and Mie resonances. Strong light–matter couplings and tunable exciton polaritons in individual nanowires are demonstrated. In addition, the excitonic responses can be reversibly modulated with excellent reproducibility, offering the potential for developing tunable optical nanodevices. Being in the mobile colloidal state with highly tunable optical properties, the TMDC nanoresonators will find promising applications in integrated active optical devices, including all-optical switches and sensors.

npj 2D Materials and Applications, Published online: 07 July 2022; doi:10.1038/s41699-022-00323-7

Evaluation of polyvinyl chloride adhesion to 2D crystal flakes

Nature, Published online: 06 July 2022; doi:10.1038/s41586-022-04863-2

The discovery of graphite–diamond hybrid carbon, Gradia, which consists of graphite and diamond nanodomains interlocked through coherent interfaces, clarifies the long-standing mystery of how graphite turns into diamond.

Light: Science & Applications, Published online: 06 July 2022; doi:10.1038/s41377-022-00900-x

Functionalizing Van der Waals materials by shaping them

Excimer formation is unambiguously demonstrated in non-van der Waals 2D semiconductor Bi₂O₂Se via transient absorption spectroscopy. The excimer in Bi₂O₂Se nanosheets is diffusive and its formation can be described as excitons relax to an excimer geometry driven by the ultrafast photoscreening of the intrinsic built-in dipolar electric field.

Abstract

The layered semiconductor Bi₂O₂Se is a promising new-type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non-vdW nature in Bi₂O₂Se with a built-in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi₂O₂Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three-stage excimer dynamics with corresponding time scales are established. Excitation-fluence-dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi₂O₂Se, which may motivate novel device designs.

A collection of machine-learning techniques is utilized to effectively discover the hidden pattern between Raman- and PL-spectra of molybdenum disulfide (MoS₂), which provide insights into the physical mechanisms connecting PL and Raman features. This study's analysis further disentangles the strain and doping contributions from the Raman spectra through machine-learning models.

Abstract

2D transition metal dichalcogenides (TMDCs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelectronic and photonic applications such as light-emitting diodes, photodetectors, and single-photon emitters. Among the standard characterization tools for 2D materials, Raman spectroscopy stands out as a fast and non-destructive technique capable of probing material's crystallinity and perturbations such as doping and strain. However, a comprehensive understanding of the correlation between photoluminescence and Raman spectra in monolayer MoS₂ remains elusive due to its highly nonlinear nature. Here, the connections between PL signatures and Raman modes are systematically explored, providing comprehensive insights into the physical mechanisms correlating PL and Raman features. This study's analysis further disentangles the strain and doping contributions from the Raman spectra through machine-learning models. First, a dense convolutional network (DenseNet) to predict PL maps by spatial Raman maps is deployed. Moreover, a gradient boosted trees model (XGBoost) with Shapley additive explanation (SHAP) to bridge the impact of individual Raman features in PL features is applied. Last, a support vector machine (SVM) to project PL features on Raman frequencies is adopted. This work may serve as a methodology for applying machine learning to characterizations of 2D materials.

Fully aerosol-jet printed (AJP) photodetectors are fabricated using megasonically exfoliated MoS₂ channels and graphene electrodes. Superlative optoelectronic performance is attributed to the megasonically thinned MoS₂ nanosheets, resulting in responsivities that exceed previous all-printed visible photodetectors by over three orders of magnitude.

Abstract

Printed 2D materials, derived from solution-processed inks, offer scalable and cost-effective routes to mechanically flexible optoelectronics. With micrometer-scale control and broad processing latitude, aerosol-jet printing (AJP) is of particular interest for all-printed circuits and systems. Here, AJP is utilized to achieve ultrahigh-responsivity photodetectors consisting of well-aligned, percolating networks of semiconducting MoS₂ nanosheets and graphene electrodes on flexible polyimide substrates. Ultrathin (≈1.2 nm thick) and high-aspect-ratio (≈1 μm lateral size) MoS₂ nanosheets are obtained by electrochemical intercalation followed by megasonic atomization during AJP, which not only aerosolizes the inks but also further exfoliates the nanosheets. The incorporation of the high-boiling-point solvent terpineol into the MoS₂ ink is critical for achieving a highly aligned and flat thin-film morphology following AJP as confirmed by grazing-incidence wide-angle X-ray scattering and atomic force microscopy. Following AJP, curing is achieved with photonic annealing, which yields quasi-ohmic contacts and photoactive channels with responsivities exceeding 10³ A W⁻¹ that outperform previously reported all-printed visible-light photodetectors by over three orders of magnitude. Megasonic exfoliation coupled with properly designed AJP ink formulations enables the superlative optoelectronic properties of ultrathin MoS₂ nanosheets to be preserved and exploited for the scalable additive manufacturing of mechanically flexible optoelectronics.